1. Field of the Invention
The present invention relates to a holographic recording medium and a holographic recording method using the same.
2. Description of the Related Art
In recent years, research has been actively conducted with respect to holographic recording, owing to its potential for high density recording, multiplexed recording, and the like. Within holographic recording, amplitude holograms based on a change in the transmittance of a recording materials, phase holograms based on a change in the refractive index, or structural irregularity of a recording material, among other techniques, are known.
Among recording materials for use in such holographic recording, much research has been conducted on photorefractive index-changing materials (hereinafter sometimes abbreviated as “photorefractive materials”) which change their refractive index when light is applied to them. In particular, organic photorefractive materials have been actively researched, since they can be easily worked into any shape and their sensitive wavelength can be easily controlled.
In photorefractive materials, charges are generated by the application of light and move to become trapped, producing an inner electric field, whereby Pockels effect takes place and thus a change in the refractive index is caused. This change in the refractive index forms a hologram. When organic photorefractive materials are used, however, orientation of the organic molecules and application of an outer electric field are required for effective production of Pockels effect. The need for an outer electric field is a significant problem when considering applications.
Holographic materials needing no outer electric field are known, a representative example of which being organic photoisomerizable materials having an azobenzene structure as a photoisomerizable group, particularly photoisomerizable polymer materials (for example, see Japanese Patent No. 2834470 and Japanese Patent Application Laid-Open (JP-A) Nos. 2001-201634, 2000-105529, 2000-109719, 2000-264962, and 2001-294652). The photoisomerization reaction of the azobenzene plays an important role in holographic recording. When linearly polarized light is applied to the azopolymer, the azobenzene is reoriented through the trans-cis-trans isomerization cycle. This reorientation induces optical anisotropy, specifically dichroism and birefringence, so that holographic recording can be performed.
In terms of factors including convenience, holographic recording is performed using a holographic recording medium that structured such that a recording layer is provided on a given substrate, board, or the like and the recording layer contains a holographic recording material. When holographic recording is performed using a photoisomerizable material, a light beam corresponding to information to be recorded is applied to the recording layer, after which the photoisomerizable material in the recording layer absorbs the light and changes its refractive index.
In the process of holographic recording, the absorption of light by the photoisomerizable material can cause a loss in holographic diffraction efficiency and can ultimately cause, for example, a reduction in reproduction output. To reduce loss in diffraction efficiency, for example, the content of azobenzene in the photoisomerizable material may be reduced. However, a low azobenzene content in the photoisomerizable material can lead to a reduction in the photosensitivity of the holographic recording material and thus can lead to an inability to carry out holographic recording itself.
The present invention has been devised in view of the above problems. The invention provides a holographic recording medium with which high diffraction-efficiency recording/reproduction can be performed independently from the optical properties of the photorefractive material, and also provides a holographic recording method using such a medium.
The invention further provides a holographic recording method by which multiplexed recording of information can be performed at high density and a high SNR (low error rate).